Powerful new autoradiographic and enzyme histochemical methods for tracing central neural connections using the physiological processes of anterograde and retrograde axoplasmic transport have been developed in the last 5 years. In addition, immunohistochemical techniques may now be used to localize certain biochemically-defined cell systems within the brain. Each of these methods have been applied in our laboratory to study the connections of the limbic system and hypothalamus, regions thought to be involved in the central regulation of the autonomic nervous system and the endocrine system. Detailed autoradiographic studies of the hippocampus, septum and preoptic area in the rat, for example, have: (a) clarified the origins of different components of the fornix system; (b) demonstrated the paths taken by visual information to the hypophysiotrophic area of the hypothalamus; and (c) revealed the efferent connections of the preoptic area to widespread regions of the brain including, among others, the olfactory bulb, the substantia nigra, and the locus coeruleus. The research proposed here is an extension of these studies and is aimed at determining: (1) the efferent connections of the dorsomedial nucleus, supramammillary region and posterior hypothalamic area with the autoradiographic and HRP methods; (2) the organization of parahippocampal region inputs to Ammon's horn and the dentate gyrus; (3) whether individual cells in paraventricular nucleus project to both the pituitary and the brainstem and spinal cord; and (4) the total population of basal forebrain inputs to the posterior lobe of the pituitary, and whether any of these projections do not contain the peptide hormones oxytocin and vasopressin.